1. Field of the Invention
This invention relates to an electrical inspection method (hereinafter simply called inspection method) for the pixel unit, that is conducted in the step of fabricating an active matrix semiconductor display device or after the completion of the active matrix semiconductor display device. More specifically, the invention relates to a method of fabricating semiconductor display devices by employing the above inspection method.
2. Description of the Related Art
In recent years, attention has been given to technology for fabricating thin-film transistors (TFTs) by using a semiconductor film of a thickness of about several to about several hundreds of nanometers formed on the surface of an insulating material to meet an increasing demand for the active matrix semiconductor display devices using TFTs as switching elements. Representative examples of the active matrix semiconductor display device may include liquid crystal display devices, light-emitting devices and DMDs (digital micromirror devices).
The active matrix semiconductor display device includes switching elements that are arranged in the pixels corresponding to several hundreds of thousand to several millions of regions divided like a matrix. The switching elements control the input of voltage or current to the semiconductor elements arranged in the pixels. Hereinafter, the voltage stands for a potential difference from a particular fixed potential unless stated otherwise.
There has recently been realized a so-called system-on-panel technology according to which drive circuits such as scanning line drive circuits for selecting the pixels and signal line drive circuits for inputting video signals to the selected pixels, are integrally formed on the same substrate of the pixel unit on where the pixels have been arranged. The system-on-panel makes it possible to greatly decrease the number of connection terminals and, hence, to decrease space for arranging the connection terminals and to increase the yield while suppressing the occurrence of defective connection.
The active matrix semiconductor display device (hereinafter simply referred to as semiconductor display device) is completed through a variety of fabrication steps. For example, a liquid crystal display device is fabricated chiefly through a step of forming a semiconductor film and forming a pattern, a step of forming color filters for realizing a color display, a step of fabricating cells by forming a liquid crystal panel by sealing liquid crystals between an element substrate having elements inclusive of a semiconductor and an opposing substrate having opposing electrodes, and a step of assembling a module by providing the liquid crystal panel assembled through the step of fabricating the cells with drive parts for operating the liquid crystal panel and with a back light thereby to complete a liquid crystal display device.
Here, the element substrate is the one in a state of before the display elements are completed in a step of fabricating the semiconductor display device.
An inspection step is often provided after the above steps though it may differ to some extent depending upon the kinds and specifications of the semiconductor display devices. If defective parts can be discriminated at an early step before the product is completed, then, the panel needs not be passed through the subsequent steps. Therefore, the inspection step is a very effective means from the standpoint of decreasing the cost.
Described below is the principle of the inspection method for confirming the operation of the pixel unit possessed by the semiconductor display device. The inspection includes three steps, i.e., accumulating an electric charge in a holding capacitor possessed by a pixel, holding the electric charge, and reading out the electric charge.
Referring, first, to FIG. 12A, a signal for inspection (hereinafter called inspection signal) is input to a signal line 1202 when a switching element 1201 possessed by a pixel is being turned on. Then, due to a current or a voltage of the inspection signal, an electric charge is accumulated in a holding capacitor 1203 provided in the pixels.
Referring, next, to FIG. 12B, the electric charge accumulated in the holding capacitor 1203 is stored therein when the switching element 1201 is turned off.
Referring to FIG. 12C, the switching element 1201 is turned on again to read the electric charge held in the holding capacitor 1203 through a signal line 1202. Relying upon the amount of electric charge that is read out, it is allowed to inspect whether the signal is normally input to the pixel and the electric charge is normally held by the holding capacitor.
In a real panel, the signal lines have not been directly connected to the connection terminals and, hence, passages are necessary for reading out the electric charge from the signal line to the connection terminal. As a passage for reading out the electric charge, a video signal line has so far been generally used.
FIG. 13A illustrates a general constitution of an element substrate of a semiconductor display device. The element substrate may be in a state where there have been completed the holding capacitors and semiconductor elements such as TFTs for controlling the accumulation of electric charge in the holding capacitors; i.e., the element substrate is in a state of before completing the display elements.
In FIG. 13A, a shift register 1211 generates a timing signal and inputs it to a sampling circuit 1212 in synchronism with a clock signal CK and a start pulse SP input to a signal line drive circuit 1210. In a sampling circuit 1212, a video signal line is electrically connected to signal lines S1 to S4 in synchronism with timing signals that are input. Hereinafter, the connection stands for an electric connection unless stated otherwise.
In the case of the element substrate shown in FIG. 13A, the electric charge can be read out from the signal line via the video signal line. Therefore, there is no need of changing the constitution of the element substrate for inspection, and the inspection is carried out relatively easily.
In recent years, however, it is a trend to use video signals of a digital form and to use an increased number of pixels, resulting in a complex connection constitution of the semiconductor elements in the pixels and a complex constitution of a signal line drive circuit, and the signal lines can no longer be simply connected to the video signal lines.
FIG. 13B illustrates the constitution of the element substrate of a semiconductor display device using digital video signals. In FIG. 13B, a shift register 1221 forms a timing signal and inputs it to a latch 1222 in synchronism with a clock signal CK and a start pulse signal SP input to a signal line drive circuit 1220. The latch 1222 latches a digital video signal input to the video signal line in synchronism with the timing signal that is input. The switching of the inverter 1223 that works as a buffer is controlled according to the digital video signal that is latched, and a power source voltage VDD or VSS (VDD>VSS) is given to the signal lines S1 to S4.
In the thus constituted element substrate, a digital video signal is input to the gates of two TFTs possessed by the inverter 1223, and the signal lines are connected to the drains of the two TFTs. Further, the video signal line is connected to the input side of the latch 1222. Here, however, the input side of the latch 1222 cannot necessarily be connected to the output side thereof. When the signal line drive circuit shown in FIG. 13B is used, therefore, it is difficult to electrically connect the video signal line to the signal lines, and it is not allowed to use the video signal line as a passage for reading the electric charge.
Therefore, a circuit dedicated to reading the electric charge (inspection-dedicated circuit) is used for inspecting the element substrate having the above constitution. FIG. 14 illustrates a state where an inspection-dedicated circuit is connected to the element substrate shown in FIG. 13A and FIG. 13B.
The inspection-dedicated circuit 1225 shown in FIG. 14 includes a sampling circuit 1227 for inspection which controls the connection of the signal lines S1 to S4 to an inspection-dedicated wiring 1228 used as a passage for reading the electric charge, and a shift register 1226 for inspection which controls the operation of the sampling circuit 1227 for inspection.
Owing to the above constitution, there is no need of using the video signal line as a passage for reading out the electric charge. Therefore, the electric charge can be read out even when the video signal line cannot be connected to the signal lines.
There, however, arouse some problems even when the inspection-dedicated circuit is used as shown in FIG. 14.
First, when the inspection-dedicated circuit is provided outside the element substrate, the inspection-dedicated circuit must be connected to the signal lines via connection terminals. Therefore, the element substrate must be provided with connection terminals for the inspection-dedicated circuit, and space for the connection terminals becomes useless after the inspection has been finished. Besides, increasing the area of the substrate for securing the place for arranging the connection terminals hinders the effort for realizing the semiconductor display devices in small sizes, and is not desirable.
When the inspection-dedicated circuit is fabricated on the same substrate as the pixel unit, further, the inspection-dedicated circuit that needs not be shipped with products becomes a factor that hinders the effort for decreasing the size of the semiconductor display devices. If the inspection-dedicated circuit were to be cut off by cutting the substrate after the inspection has been finished, the element substrates are obtained in a decreased number from a piece of large substrate which is a mother glass due to space occupied by the inspection-dedicated circuit.